The present invention relates generally to the data processing field, and more particularly, relates to a method and apparatus for in situ detection of high-flying sliders over customer data in a direct access storage device (DASD).
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads mounted on air bearing sliders, are driven in a path toward and away from the drive axis, write data to the disks and read data from the disks.
In disk drives or direct access storage devices (DASDs), the continuing increases in data storing capacities are accomplished by increasing linear recording densities and track densities. To provide an adequate signal-to-noise ratio (SNR) of the readback signal, the nominal head to disk spacing is reduced. This decreased head to disk spacing can significantly increase the occurrences of head to disk interactions and the probability of a terminal head crash. To minimize the effects of head to disk interaction and to increase the reliability of the disk drive, surface lubricant is used.
At times during normal disk-drive operation, temporary increased head-to-disk spacing is caused, for example, by excessive buildup of disk surface lubricant at random locations on the head and disc surfaces. An excessive amount of lubricant causes a temporary increase in head-to-disk spacing and can result in data errors. A high-flying slider carrying the transducer heads often causes data errors due to increased attenuation of the high-frequency components both in the recorded or written signal and in the readback signal.
Transducer heads or read/write heads typically include a thin film inductive head or write head for writing data and a magnetoresistive (MR) head or read head for reading data. The customer data is written or recorded on the disk as variable-length regions of alternating magnetized media, for example, alternating north-south magnetization. Thus, the readback signal would ideally resemble a variable frequency square wave signal which is very rich in harmonic frequencies. Both the MR read head and thin-film inductive write head are subject to head-to-disk spacing attenuation or loss. The head-to-disk normalized spacing-loss Q(f1) is a negative exponential function of the head-to-disk spacing (d). The well known Wallace""s equation gives the spacing loss equation Q(f1) for a spatial frequency f1 as:
Q(f1)=exp(xe2x88x922*xcfx80*d/lam1)xe2x80x83xe2x80x83Equation (1)
where d is the head-to-disk spacing (meters) and lamI=1/f1 is the spatial wavelength (meters) of the spatial frequency fi. The spatial wavelength can be found from the relation:
lam1=xcfx80*r*RPM/30*f1xe2x80x83xe2x80x83Equation (2)
where r is the track radius, RPM is the spindle velocity, and f1 is the recording frequency (Hz).
If we designate f1 as an all ones recording frequency or a (1 0 1 0 1 0 1 . . . ) frequency and its third harmonic frequency as f3=3*f1, then the ratio R of the normalized spacing loss at f1 and f3 can be found from Equation (1) to be equal to:
R=Q(f1)/Q(f3)=exp(4*xcfx80*d/lam1)xe2x80x83xe2x80x83Equation (3)
Various flyheight estimation techniques are known that employ Equation (1) and Equation (3). For example, solving Equation (1) for the flyheight separation d provides the basis for a known clearance modulation detection (CMD) method, and solving Equation (3) for the flyheight separation d provides the basis for another harmonic ratio flyheight (HRF) method. Both the CMD method and HRF method have proved very valuable in the detection and estimation of flyheight variation. The main problem with these methods is that they require a readback signal with a single frequency to function. Thus, the prior art methods do not work when flying over customer data where the readback produced is a frequency modulated signal. At the present time, there are no known in situ methods for monitoring the flyheight of a slider while reading customer data.
A need exists for a method and apparatus for in situ detection of high-flying sliders over customer data. It is desirable to provide such method and apparatus for in situ detection of high-flying sliders while the slider is reading and writing customer data.
A principal object of the present invention is to provide a method and apparatus for in situ detection of high-flying sliders over customer data. Other important objects of the present invention are to provide such method and apparatus for in situ detection of high-flying sliders over customer data substantially without negative effect and that overcome some disadvantages of prior art arrangements.
In brief, a method and apparatus are provided for in situ detection of high-flying sliders over customer data. A sampled and digitized readback sequence x(n) from a logical data block of customer data is received. Bandpass filtered data of the readback sequence x(n) is generated and processed for generating a predefined harmonic ratio. The generated predefined harmonic ratio is compared with a predefined threshold value to identify a high-flying slider.
In accordance with features of the invention, bandpass filtered data of the readback sequence x(n) is generated with a digital comb filter and a digital resonator. The bandpass filtered data of the readback sequence x(n) is either squared or the absolute value is taken for a predefined number of samples and then accumulatively summed for the predefined number of samples and the harmonic ratio is calculated. The generated predefined harmonic ratio is compared with a predefined threshold value that is determined as a function of head and disk parameters. During a read operation, a high-frequency boost is applied by using a read channel equalizer responsive to identifying a high-flying slider. During a write operation to a selected write location, the write operation is aborted responsive to identifying a high-flying slider near the selected write location.